console.log(1+undefined);
console.log(1+null);
console.log(NaN + 1)
console.log([]+1)
console.log(null + undefined)
const a = [1,2,3];
console.log(a.valueOf())
console.log(a.toString())

const b = [4,5];
console.log(a + b);               // "1,2,34,5"